Process for the manufacture of sorbic acid



United States Patent Office 3,021,365 Patented Feb. 13, 1962 3,021,365 PROCESS FOR THE MANUFACTURE OF SORBIC ACID Hans Fernhoiz, Bad Soden (Taunus), and Eberhard Mundlos, Frankfurt am Main, Germany, assignors to Farbwerke Hoechst Aktiengesellschait vormals Meister Lucius & Briining, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed June 3, 1957, Ser. No. 662,968 Claims priority, application Germany June 7, 1956 7 Claims. (Cl. 260-526) The present invention relates to a process for the manufacture of sorbic acid.

It has already been proposed to prepare sorbic acid by adding ketene to crotonaldehyde in the presence of an inert solvent and an acid catalyst, preferably a borotrifiuoride-catalyst, to obtain a lactone which can be converted to sorbic acid by treating said lactone with a strong acid.

Now we have found that the reaction of ketene with crotonaldehyde in the presence of a catalytic amount of a fatty acid salt of a bivalent transition metal leads to a product which cannot, or can only partially, be converted to sorbic acid by treating said product with a strong acid as above described, this being surprising. Said product enables, however, without previous separation of the catalyst used, sorbic acid to be obtained in a better yield as compared with the known process, by first hydrolyzing said product in an alkaline medium and then treating it with a strong acid.

Accordingly, the process for the manufacture of sorbic acid according to this invention consists in reacting ketene with crotonaldehyde at a temperature within the range of 20 C. and 50 C., in the presence of a fatty acid salt of an at least bivalent transition metal and in the presence of a solvent inert towards the reaction, in removing the solvent and the excessive aldehyde, if any, by distillation, in hydrolyzing the polymeric and catalyst-containing product obtained in an alkaline medium, and in converting the resulting alkaline solution after the addition of at least one strong acid to sorbic acid at a raised temperature. a

The first stage of the present process leads to a new product which obviously differs from the lactone obtained in the known process and whose properties indicate an ester with a molecular weight of between about 1000 and 3000. Said product is probably a polyester which is obtained during the reaction of ketene with crotonaldehyde from the beta-lactone intermediarily formed. Said polyester which is obtained in a practically quantitative yield by the process of this invention cannot, or can only partially, be converted to sorbic acid by treating it with a strong acid.

, As catalysts suitable for use in this invention there may be used fatty acid salts of bivalent transition metals, such as cadmium, iron, nickel, mercury, cobalt or zinc, or mixtures of these salts, provided that the fatty acid radical contains 448 carbon atoms. in view of the fact that sorbic acid is often used for the preservation of food, it is advisable to use compounds of transition metals non-toxic for human beings so as to exclude any risk in case the final product obtained should still contain traces of said metals. It is especially advantageous to use zinc salts of fatty acids, especially salts of zinc isobutyrate or zinc isovalerate. There may also be used salts of butyric acid, valeric acid, alpha-methylbuytric acid, diethylacetic acid, caproic acid, caprylic acid, capric acid, lauric acid, Z-ethyl caproic acid, stearic acid, palmitic acid, oleic acid or other fatty acids containing 4-18 carbon atoms. The catalysts are ge erally used in a proportion of between 0.1 and 5%, preferably between 0.5 and 2%, calculated may also be used in a quantity outside the ranges indicated above.

As suitable solvents there may be used aromatic, aliphatic and alicyclic hydrocarbons or the derivatives thereof and excessive crotonaldehyde. There may be mentioned more especially: Hexane, heptane, octane, benzene, toluene, xylene, cyclohexane, methylene chloride, chloroform, carbon tetrachloride, chlorobenzene or nitrobenzene. It is often preferred to use an equimolecular quantity of ketene and crotonaldehyde in which the catalyst is dissolved. In this case the reaction is not entirely complete because a small proportion of the crotonaldehyde acts as solvent and does not react. It is also preferred to carry out the reaction at a temperature between 25 and 30 C.; it is, however, also possible to operate at a raised temperature, if desired, although such operation generally leads to less favorable results. If the first stage of the reaction is carried out at a term perature lower than about 20 C., the reaction proceeds slowly and incompletely.

In subjecting the polyester to an alkaline hydrolysis, preferably between 60 and 100 C. which may easily be carried out, for example by heating to l00'C. within a period of some minutes up to 1 hour using a 1040% sodium hydroxide solution, there is obtained a clear brownish solution which substantially contains salts of unsaturated hydroxycarboxylic acids corresponding to the bases used. The major quantity of free hydroxy acids precipitates upon the addition of acid. These free acids may be dehydrated by treating them in the hot with a strong aqueous acid, preferably at a temperature within the range of 80150 C., for example below C., and by heating for 1 hour to 90-100 C. in a 20-30% sulfuric acid or in a 15-25% hydrochloric acid. The acid treatment may, if desired, also be carried out under superatrnospheric pressure. This operation leads to a crude light brown to yellow sorbic acid which is ob tained in a practically quantitative yield and from which about 90% of pure colorless sorbic acid may be obtained.

For hydrolyzing the polymerization product obtained in the first stage of the process there may especially be used strong bases, such as sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide and barium hydroxide solution. For the acid treatment of the solutions obtained with the aforesaid lyes there may be used strong acids, for example hydrochloric acid, sulfuric acid, benzene sulfonic acid, para-toluene sulfonic acid or mixtures of these acids.

It has further been found that still better results are obtained by subjecting the catalyst-containing polyester obtained in the first stage of the process to a thermal conversion instead of to an alkaline hydrolysis and a subsequent acid treatment.

The yields obtained by the present process are considerably higher than those which are obtained by a thermal decomposition of the known lactone under identical conditions. It is suitable to carry out the thermal conversion at a temperature between and 220 C., preferably between and 210 C.; it is, however, also possible to operate at a temperature outside the ranges indicated above, although such procedure generally does not lead to better results. If, for example, the polyester is heated for some minutes to about 200 C., an exothermic decomposition reaction is initiated and about 60% of pure sorbic acid can be obtained from the decomposition product. Better yields of sorbic acid are obtained if the polymeric ketene-crotonaldehyde product is therupon the quantity of crotonaldehyde, but sometimes they mally decomposed in the presence of a catalytic amount, preferably 15%, of a substance having an alkaline reaction. As substances of this type there may be mentioned, for example bases, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or also alkali metal salts having an alkaline reaction, preferably salts with organic acids having an acidity lower than sorbic acid, such as sodium or potassium carbonate, isobutyrate, caproate, caprylate, palmitate or stearate or the corresponding calcium or barium salts or also potassium or sodium sorbate.

It has also been found that the thermal decomposition leads to considerably better results by mixing the polymeric catalyst-containing keteue-crotonaldehyde addition product with another substance inert towards the thermal decomposition and boiling at a temperature above 150 C. and preferably above 180 C. under atmospheric pressure, and heating the mixture so obtained to a temperature above 150300 C. preferably above ISO-270 C. In this case, too, the presence of a small amount of an alkali metal or an alkaline earth metal compound having a distinct basic reaction, for example a compound of the aforesaid metals, displays a favorable efiect. These sub-. stances are generally used in a proportion of 1-15 preferably l%, calculated as metal hydroxide upon the quantity of the ketene-crotonaldehyde reaction product. If the reaction of ketene with crotonaldehyde is carried out in a solvent being also suitable for the decomposition, the solution obtained may directly be subjected to a thermal decomposition after evaporation of excessive crotonaldehyde, if any, and, if desired, in the presence of a substance having an alkaline reaction as catalyst. This operation may, of course, only be carried out with those of the below mentioned solvents which are liquid at the reaction temperature of 205 0 C.

As diluents suitable for use in the thermal decomposition there may be mentioned aliphatic, alicyclic and aromatic hydrocarbons, the chloro-, bromoand nitroderivatives thereof, and also ethers and silicon oils boiling within the range of 150 and 300 C. and preferably above 180 and 270 C. under atmospheric pressure. As diluents there may also be used ketones, esters, carboxylic acids and alcohols with a corresponding boiling range although their use generally leads to less favorable results, because these diluents may partially react with the reaction mixture. It is preferred to use diluents or solvents which are liquid at normal temperature and from which pure sorbic acid precipitates upon cooling. It is especially advantageous to use substances which are capable of forming azeotropic mixtures with sorbic acid so that they may be used at the same time as a carrier or an entraining agent. There may be mentioned more especially: Aliphatic, alicyclic or aromatic hydrocarbons, such as petroleum fractions, dodecane, tetradecane, S-methyl-dodecane, dodecene, dicyclohexyl, dicyclohexylmethane, para-ditertiary-butylbenzene, l-methylnaphthalene, 2- methylnaphthalene, l-ethylnaphthalene, tetrahydronaphthalene, diphenyl, naphthalene; halogenated aliphatic, cycloaliphatic or aromatic hydrocarbons, such as dichlorododecane, 1.5-dibromopentane, benzotrichloride, orthoand meta-dibromobenzene; nitro-compounds, such as nitrobenzene, 2-nitrotoluene; nitriles, such as benzyl cyanide; carbonyl compounds, such as acetophenone, dihydrocarvone or heterocyclic Z-acetylthiophene; heterocyclic compounds, such as chromane, and thiophthene; ethers, such as resorcinol dimethylether, diphenylether, safrol, isosafrol; carboxylic acids, such as enanthic acid, alpha-ethylcaproic acid, caprylicacid, capric acid; esters, such as succinic aciddiethyl ester, glutaric acid diethyl ester, benzoic acid. ethyl ester, phenyl acetic acid methyl ester, salicylic acid methyl ester; alcohols, such as diethylene, glycol, glycerol, ethylene glycol, propylene glycol; monoalkyl ethers ofglycols in which the, alkyl groups contain 1-3 carbon atoms; ethylhexanol and octanol. The thermal decomposition may, in principle, also be carried out. under superatmospheric pressure provided that relatively readily boiling diluents are used boiling in the range indicated above and provided that the presence of great amounts of diluent in additoin to small amounts of sorbic,

acid is immaterial when the distillation is carried out simultaneously.

The decomposition itself may advantageously be brought about in vacuo. The diluent which generally acts as a solvent is added in a quantity of 115, preferably 1-10 parts by weight per part by weight of ketenecrotonaldehyde reaction product; there may, however, also be used mixtures containing less than 1 part or more than 15 parts at diluent; In many cases it is especially advantageous to purify the mixture of sorbic acid and diluent obtained during the decomposition by direct or subsequent distillation. By mixing 1-10 parts by weight of diluent with 1 part of polymeric catalyst-contaimng ketene-crotonaldehyde product and heating this mixture to a temperature as indicated above in the presence of a catalytic amount of alkali metal hydroxide or alkali metal salts having an alkaline reaction, sorbic acid is obtained almost immediately which may be removed at the same time, if desired in vacuo, together with the diluent by distillation, so that the thermal decomposition of the ketenecrotonaldehyde adduct and the purification of sorbic acid are carried out in one single operation. Analytically pure sorbic acid which may be used as a preservative is obtained in a yield of more than Sorbic acid may be isolated for example by filtration of the distillate. The V separated diluent or carrier which generally contains a further amount of sorbic acid may be recycled and used again, so that the process of this invention becomes still more economic. After conversion or distillation, it is possible to add further and more volatile solvents, sucli as carbon tetrachloride, petroleum ether or cyclohexane to the mixture of sorbic acid and diluent or, alternatively, Waterprovided that solvents miscible with. water are usedso as to improve the separation of sorbic acid from the above mixture. It is likewise possible to free sorbic acid from adhering diluent using the aforesaid easily volatile solvents in which sorbic acid is scarcely soluble.-

The above process, especially the thermal dec mpose tion as compared with the known process constitutes a considerable improvement in the manufacture of sorbic acid from reaction products of ketene with crotonaldehyde. An advantage offered by the thermal decomposition resides in the fact that considerable amounts of the otherwise necessary strong acids are saved and that it is easier to carry out so that the thermal decomposition is of special commercial interest for the manufacture of sorbic acid on an industrial scale.

The following examples serve to illustrate the invention but they are not intended to limit it thereto:

Example 1 420 grams of ketene were introduced at a temperature between 25 and 35 C. into a mechanically stirred or moved mixture of 800 grams of crotonaldehyde, 1200 cc. of toluene and 8 grams of zinc isovalerate. The croton aldehyde in excess (100 grams) and the toluene were removed in vacuo. As residue there were obtained 1150- 1250 grams of a polyester in the form of a highly viscous brown liquid containing a small amount of toluene. The total quantity of the product so obtained was mixed with 1400 grams of a 35% sodium hydroxide solution and at the end of the exothermic reaction with 1400 grams of water. The resulting mixture was then heated for one hour to -100" C. while stirring to give a clear brown solution. The solution was allowed to cool and 2900 grams of an 80% sulfuric acid were then added while stirring and the whole was heated for another hour to 90100 C. while stirring. After cooling, there were obtained 1100 grams of an about 90% crude sorbic acid which in most cases had a light brown coloration. It can be purified, for example, by distillation with a glycol or a monoalkylglycol ether.

Example 2 grams of a product (polyester) obtained by re acting equivalent amounts of ketene and crotonaldehyde in the presence of toluene (crotonaldehydeatoluene=1:1.2) and zinc isovalerate 1% of the amount of crotonaldehyde used) and freed from toluene, were heated to 195 C. in a flask provided with a cooler. A vigorous reaction set in while the temperature was kept at 205 C. The reaction was complete after about minutes. The decomposition product was mixed with three times the quantity of diethylene glycol and subjected to a distillation in vacuo. Pure colorless sorbic acid was obtained in a yield of 62 grams.

Example 3 100 grams of the polyester of Example 2 were mixed in each case with (a) 24 grams of caustic soda (b) 2-4 grams of caustic potash (c) 4 grams of sodium sorbate treated and worked up as described in the preceding example. The following yields of sorbic acid were obtained:

Example 4 A mixture of 400 grams of tetrahydronaphthalene, 100 grams of a polymeric catalyst-containing ketene-crotonaldehyde product (referred to hereinafter as polyester) and 3 grams of sodium hydroxide was refluxed for 30 minutes. The mixture was allowed to cool, then filtered while applying reduced pressure, and the residue washed with cyclohexane. An about 95% brownish sorbic acid was obtained in a yield of 90 grams. It melted at l3013l C.

Example 5 A mixture of 300 grams of parafiin oil, 100 grams of a polyester (see Example 4) and 6 grams of sodium stearate was heated for 1 hour to 2l0220 C., diluted with 100 cc. of petroleum ether, filtered while applying reduced pressure, and the residue Washed with petroleum ether. A slightly brownish colored sorbic acid was obtained in a yield of 92 grams. It melted at 131 C.

Example 6 A mixture of 500 grams of l-methylnaphthaleue, 100 grams of a polyester (see Example 4) and 6 grams of sodium sorbate was heated to 200-210 C. and simultaneously distilled off under sufliciently reduced pressure. From the distillate which passed over at l90l95 C. there were obtained 82 grams of analytically pure sorbic acid. It melted at 134-135 C.

Example 7 A mixture of 600 grams of a petroleum fraction boiling between 200 and 250 C. under atmospheric pressure, 100 grams of a polyester (see Example 4) and 8 grams of sodium stearate was distilled under atmospheric pressure. By filtering with application of reduced pressure and washing the residue with petroleum ether there were obtained 81 grams of analytically pure sorbic acid.

Example 8 A mixture of 300 grams of 2-nitrotoluene, 100 grams of a polyester (see Example 4) and 7 grams of sodiumalpha-ethyl-caproate was distilled under atmospheric pressure. The distillate to which were added 200 grams of carbon tetrachloride passed over at 220225 C. There were obtained 69 grams of pure sorbic acid melting at 134 C.

Example 9 A mixture of 400 grams of ortho-dibromohenzene, 100 grams of a polyester (see Example 4) and 6 grams of curd soap was distilled under atmospheric pressure. The

6 distillate to which were added 150 cc. of carbon tetrachloride passed over at 224230 C. There were obtained 72 grams of analytically pure sorbic acid.

Example 10 A mixture of 300 grams of resorcinol-dimethyl ether, grams of a polyester (see Example 4) and 5 grams of soft soap was distilled under atmospheric pressure. The distillate to which were added 200 cc. of cyclohexane passed over at 210-220 C. There were obtained 74 grams of pure colorless sorbic acid.

Example 11 Example 12 A solution of 100 grams of a polyester (see Example 4) and 4 grams of sodium hydroxide in 350 grams of diethylene glycol was introduced dropwise into a flask provided with a descending cooler and a receiver and heated on an oil bath to 170-l80 C., and then distilled under reduced pressure (l0ll mm. of mercury). The distillate separated 50 grams of analytically pure sorbic acid. By the addition of 500 grams of water there were obtained another 28 grams of sorbic acid.

We claim;

1. A process for the manufacture of sorbic aeid, which comprises reacting ketene with croton aldehyde at a temperature in the range from 20 C. to 50 C. in the presence of an inert solvent and in the presence of a salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and zinc, distilling ofi vaporizable compounds and thermally decomposing the residue which still contains said fatty acid salt.

2. A process for the manufacture of sorbic acid, which comprises reacting ketene with croton aldehyde at a temperature in the range from 20 C. to 50 C. in the presence of an inert solvent and in the presence of 0.1 to 5 percent, calculated upon the amount (by weight) of croton aldehyde, of a salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and zinc, distilling oft vaporizable compounds, hydrolyzing the residue still containing said salt of a fatty acid in an alkaline medium, and converting the hydrolyzed product by treatment with an aqueous solution of a strong acid at a temperature in the range from 80 C. to C. to sorbic acid.

3. A process for the manufacture of sorbic acid, which comprises reacting lcetene with croton aldehyde at a temperature in the range from 20 C. to 50 C. in the presence of an inert solvent and in the presence of a salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and Zinc, distilling ofi vaporizable compounds and thermally decomposing the residue in the presence of the fatty acid salt and of an alkaline compound selected from the group consisting of hydroxides and salts of alkali metals and alkaline earth metals, said salts being salts of an acid weaker than sorbic acid.

4. A process for the manufacture of sorbic acid, which comprises reacting ketene with croton aldehyde at a temperature in the range from 20 to 50 C. in the presence of an inert solvent and in the presence of a salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and zinc, distilling off vaporizable compounds and thermally deco-mposing the residue at a temperature between 150 and 300 C. in the presence of the fatty acid salt and of an inert diluent boiling under normal pressure at a temperature in the range from 150 C. to 300 C.

5. A process for the manufacture of sorbic acid, which comprises reacting ketene with croton aldehyde at a temperature in the range from 20 to 50 C. in the presence of an inert solvent and in the presence ofa salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and zinc, distilling 01f vaporizable compounds and thermally decomposing the residue at a temperature between 150 C. and 300 C. in the presence of the fatty acid salt and of 1 to percent of an alkaline compound selected from the group consisting of hydroxides and salts of alkali metals and alkaline earth metals, said saltsbeing salts of an acid weaker than sorbic acid, and in the presence of an inert diluent boiling under normal pressure at a temperature in the range from 150 to 300 C.

6. A'process for the manufacture of sorbic acid, which comprises reacting ketene with croton aldehyde at a temperature in the range from to 50 C. in the presence of an inert solvent and in the presence of a salt of a fatty acid containing four to eighteen carbon atoms and a bivalent transition metal selected from the group consisting of cadmium, iron, nickel, mercury, cobalt, and zinc, distilling an vaporizable compounds and thermally decomposing the residue which still contains said fatty acid salt at a temperature between 150 and 300 C. and simultaneously distilling the diluent together with the soroic acid formed. I v

7. A process for the manufacture of sorbic acid, which comprises reacting ketene with croton aldehyde at a temperature in the range from 25 to C. in the presence of an inert solvent and in the presence of 0.1 to 5 percent, calculated upon the amount of croton aldehyde, of a zinc salt of a fatty acid containing four to eighteen carbon atoms, distilling 01f the vaporizable compounds and thermally decomposing the residue containing still said zinc salt in the presence of at leastone hydrocarbon boiling between and 300 C. under normal pressure and in the presence of 1 to 15 percent of an alkaline compound selected from the group consisting of hydroxides and salts of alkali metals and alkaline ear'thmetals, said salts being salts of an acid weaker than sorbic acid.

References Cited in the'file of this patent OTHER REFERENCES Caldwell: Abstract of application Serial No. 252,194, I published August'18, 1953,673 O.G. 839. 

1. A PROCESS FOR THE MANUFACTURE OF SORBIC ACID, WHICH COMPRISES REACTING KETENE WITH CROTON ALDEHYDE AT A TEMPERATURE IN THE RANGE FROM 20*C. T O 50*C. IN THE PRESENCE OF AN INERT SOLVENT AND IN THE PRESENCE OF A SALT OF A FATTY ACID CONTAINING FOUR TO EIGHTEEN CARBON ATOMS AND A BIVALENT TRANSITION METAL SELECTED FROM THE GROUP CONSISTING OF CADMIUM, IRON, NICKEL, MERCURY, COBALT, AND ZINC, DISTILLING OFF VAPORIZABLE COMPOUNDS AND THERMALLY DECOMPOSING THE RESIDUE WHICH STILL CONTAINS SAID FATTY ACID SALT. 